Growth cones integrate signaling information from a multitude of guidance molecules during development and regeneration. Distinguishing between cue/receptor interactions that underlie significant directional changes and those that do not is expedited by studying choice points at which two nerves diverge. An excellent system to investigate divergence is provided by the rodent geniculate ganglion. The peripheral nerves diverge first at the ganglion which, combined with the large scale and modest axon density of the peripheral nervous system, facilitates labeling of neuronal sub-populations and assessment of path-finding defects at early stages of development. Two of the three major geniculate nerves are homofunctional (destined to innervate taste buds in the palate or tongue), suggesting that a higher proportion of the gene expression differences between the neuronal populations will be related to the divergence. However, there is already evidence of heterogeneity in the expression of receptors for neurotrophic factors and guidance cues in geniculate neurons, and the relevance of this heterogeneity to path-finding differences will be tested. This feasibility study combines new and established procedures that take advantage of the unique features of the geniculate ganglion to determine if axons of divergent nerves differ in their responses to neurotrophic factors and guidance cues in vitro and in situ, and to determine if receptors for these factors are differentially expressed in nerve-specific sub-populations of geniculate neurons. Approaches: Neuronal somata will be retrogradely labeled by applying fluorescent lipophilic dyes to divergent nerves in live and fixed embryos. 1. Live ganglia will be cultured and the influence of the guidance factors on the morphology and direction of regenerating, labeled axons will be assessed. 2. Fixed ganglia will be sectioned and single labeled neurons will be micro-dissected for PCR-based amplification of mRNAs encoding cue receptors. 3. An in vitro organ culture method was developed in which nerve development proceeds as in vivo. This will be used to test the influence of candidate guidance cues on in situ peripheral nerve growth. 4. Mutant mice lacking the genes encoding putative divergence- relevant cues will be examined to determine which steps of path-finding depend on the missing gene product.